Vapour Lock generally occurs when the fuel inside the fuel delivery system overheats and vaporizes rapidly, the resulting excess gas bubbles accumulate to a level that blocks normal fuel delivery. Vapour lock is caused in warmer temperatures as the fuel can vaporize in lines when it receives heat from the engine, exhaust and/or outside temperature. High altitude and other factors can lower the fuels boiling point, leading to vapour lock. Vapour Lock is a modern problem, since fuel was designed for modern fuel injection systems, Fuel-injected engines use a more volatile fuel that is easier to vaporize and mix with air for a much better combustion. So this new fuel has a lower boiling point, this vaporized fuel can cause back pressure in the fuel system to prevent fuel from reaching your engine, this happens when the motorvehicle is idling or when it has been turned off and back on. Vapour locks are prevented in modern engines because of the use of an in-tank, electric fuel pump. This allows the pump to operate at a low point in the tank, submerged in fuel, and under a lower temperature than the older mechanical pump located in the engine compartment. Also, in a modern fuel delivery system fuel is pressurized.Furthermore, modern engines are equipped with an electric cooling fan, making it even harder for vapor bubbles to form in the fuel lines under high ambient or operating temperatures. Still, a modern fuel-injected system can suffer from vapor lock, not only if pressure in the fuel line drops (faulty fuel pressure regulator or fuel pump, for example) and the fuel line is subjected to high temperatures, but also modern engines that may reach between 120 and 140 C using ethanol blended fuels.